The High Court is set to begin delivering a verdict on whether 16 opposition figures breached the national security law by participating in an unofficial legislative primary election four years ago.

Queues formed outside the West Kowloon Court where the hearing is held amid tightened security and a heavy police presence.

Sixteen defendants who have pleaded not guilty are expecting to hear their fate after a subversion trial lasting 118 days.

If convicted, they could face up to life in prison although sentencing is expected later.

They were among the 47 politicians and activists charged with conspiracy to commit subversion under the national security law.

Authorities alleged that an unofficial Legco primary election in July 2020 was a subversive attempt by the opposition camp to win a majority in Legco and paralyse the government.

The 16 who stood trial include former lawmakers Lam Cheuk-ting and Leung Kwok-hung, and journalist-turned-activist Gwyneth Ho.

The case was centred on whether they conspired to subvert state power by planning to indiscriminately vote down the budget and force the then-Chief Executive to resign through their participation in the polls.

Thirty-one others, including former legal scholar Benny Tai and ex-student activist Joshua Wong, pleaded guilty to their charges.

The judiciary has set aside more than 400 seats for the public to hear the verdict on Thursday - the largest national security case to date.

Three national security judges - Andrew Chan, Alex Lee and Johnny Chan - presided over the jury-free trial.